1. Field of the Invention
The present invention relates to a plasma generating device which is capable of stably discharging under atmospheric pressure and producing low-temperature discharging plasma, and a method of plasma processing, particularly a method of etching a substrate without heating the substrate under atmospheric pressure by using such a plasma generator.
A plasma generating device of the present invention is also applicable to a fine processing by confining plasma in a minute region of a cylinder.
2. Description of the Prior Art
An atmospheric condition is an insulator in a low electric field, but when a high electric field such as direct current, alternating current and impulse is applied, a breakdown is caused, so that an electric current flows (self-maintaining discharge). The self-maintaining discharge is divided into a corona discharge, a glow discharge and an arc discharge. In a uniform electric field, the atmospheric condition entirely breaks down immediately after it shifts to a self-maintaining discharge, and then shifts to a glow discharge or an arc discharge. In contrast, in a nonuniform electric field, only a portion having a strong electric field first causes a breakdown and then a corona discharge is generated. Thereafter, when the electric field is further intensified, the discharge grows to an entire breakdown. In the atmosphere, when the discharge shifts to an entire breakdown, it generally tends to shift rapidly to an arc discharge without a process of a glow discharge. This is considered to be because although the characteristic feature of an arc discharge is a thermionic emission (existence of a cathode spot) due to an electrode heating caused by incident ions, since a larger number of ions enter the electrode under a high pressure than under a low pressure, the electrode is heated in a very short time, thereby causing a thermionic emission. Even if the electric current is not more than 2 A, a glow discharge is sometimes used, but its controllability is not good, so that an arc discharge is preferably applied to welding, cutting or the like.
Since the arc discharge is applied to melting or fusing of a workpiece by using the high temperatures of the electrode and the positive column, the temperature of the portion processed rises as high as 2000 to 6000 K. It is therefore impossible to process a workpiece without heating.
In order to enable a surface treatment, surface processing at a room temperature, various attempts to stably generate a glow discharge under atmospheric pressure have been made (S. Kanazawa et. al. J. Phys. D:Appl. Phys. 21(1988)838-840). The conditions which are necessary for generating a stable glow discharge under atmospheric pressure are (1) that the discharge space is filled with He, (2) that an insulator is inserted between the electrodes (in a discharge path), (3) that at least one of the electrodes has a shape of a needle or a brush, (4) and that the frequency of the electric field applied is not less than 3 kHz. The insulator is inserted so as to prevent the discharge from shifting to an arc discharge. The frequency of the electric field applied is set at not less than 3 kHz in order to make an electric current flow through the insulator. It is in order to make the initiation of discharge easier by making the electric field a nonuniform electric field that the electrode is caused to have a shape of a needle or a brush. Surface treatment such as etching of an organic matter such as polyimide or an inorganic matter such as silicon has also been made by utilizing these methods. These methods, however, have to go through a process of evacuating the reaction space and thereafter filling up the space with helium, although they are carried on under atmospheric pressure. Furthermore, these methods have a disadvantage in that the treatment must be uniformly carried on the substrate, so that it is impossible to selectively treat a minute region.
In order to solve the above-described problems, the present inventors invented a plasma generator and an etching method using the apparatus which is capable of processing, surface treatment and etching of a minute region without the need for a step of evacuating the reaction space by generating a stable low-temperature plasma and confining the plasma in the minute region. (Japanese Patent Application No. 286883/1990)
The method comprises the steps of concentrically providing electrodes which are composed of an electric conductor such as metal, inserting a cylindrical insulator in the gap between the electrodes in such a manner as to be concentric with the electrodes and to come into contact with the outer electrode, keeping a gas containing helium as the main ingredient flowing in the gap between the insulator and the center electrode under atmospheric pressure, applying an alternating electric field between the electrodes so as to ionize the gas containing helium as the main ingredient, thereby generating plasma in the gap between the insulator and the center electrode, and introducing an etching gas into the plasma.
Such plasma generated under atmospheric pressure, however, has a high probability of particle collision (that is, the mean free path is short) under this atmospheric pressure, so that the probability of electrons and ions recombining in the space also becomes high. As a result, plasma does not spatially extend and is rapidly canceled with the decrease of the potential gradient, so that only a few radicals reach the substrate. In order to prevent this, it is necessary to bring the substrate as close as possible to the discharge area, but since the radical density greatly changes even with a slight change of distance in the vicinity of the discharge area, the deterioration of its controllability remained a problem.